Recently, concurrently with popularization of navigation systems, the number of vehicles equipped with a monitoring system, which displays surroundings of a vehicle on a display monitor installed in a vehicle compartment, is increasing. The surroundings of the vehicle, for example, positions of other vehicles, conditions of obstacles, information of road signs such as center lines and stop lines, are displayed on a display apparatus of such as a back monitor utilized when moving the vehicle backward, a monitor for monitoring under a front part of a bumper, and a blind corner monitor for monitoring corner portions. Such monitoring systems are designed to reduce burden on a driver. Accordingly, considering large number of information desired by the driver to obtain while driving the vehicle, it is preferable that the monitoring system has good visibility in images displayed on the display apparatus, which enables a driver to obtain information desired while driving the vehicle at a glance without feeling strangeness. Further, it is naturally preferable that the images have wide field of view. Generally, field of view of human for a static object is approximately 200° when human focuses on a single point with his both eyes. It is said that a field of view of human in which human can recognize colors such as red, blue, and yellow is up to approximately 70°. Further, field of view of human when the human sees objects while he is moving decreases as the human moves faster, down to approximately 100° when the human is moving at the speed of 40 km/h, approximately half of the field of view of human for static objects. To avoid the field of view becoming narrow, drivers give much effort to maintain wide view angle by avoiding staring at a point. It is desired that the monitoring system described above preferably compensates such physiological limit of the driver or helps such effort thereof.
However, a view angle of images taken by one camera is generally narrow, approximately 50° to 60°. Accordingly, only installing one camera to the vehicle cannot result in, sufficient width of the view angle. For overcoming such problem, various methods are proposed. For example, as illustrated in FIGS. 11 and 12, there is a conventional method for taking images of places invisible from a driver's seat and displaying the images in separate display frames on a display apparatus.
Further, JPH3-99952A describes another conventional monitoring system. The monitoring system includes one camera or plural cameras, means for converting a coordinate of an image transmitted from the camera(s) into another coordinate by means of perspective conversion, a means for synthesizing a single image from the converted images with considerations in a relation to the vehicle, a display (display apparatus) for displaying the images to an occupant (driver) of the vehicle. By the perspective conversion described in this document, the image(s) taken by the camera(s) is converted into a road surface coordinate (plane coordinate), of which an origin is taken from the center of the vehicle, x-axis is taken on a line in left to right direction relative to forward direction of the vehicle, y-axis is taken along the forward direction of the vehicle. Then, the synthesized image is displayed on the display apparatus. In the synthesized image, an illustration of own vehicle is displayed. Further, other vehicles, road signs and/or objects indicated by the plane coordinate are displayed.
The perspective conversion is also called as a viewpoint conversion. As seen in the document described above, by this conversion, a screen coordinate of the camera seen from a direction parallel to a X-Y plane configured from the X-axis and the Y-axis described above is converted into the plane coordinate seen from a direction perpendicular to the X-Y plane. Accordingly, such as poles and walls, objects standing upright on the X-Y plane, can be seen when they are displayed by the screen coordinate, but cannot be seen when they are displayed by the plane coordinate. In the plane coordinate, such objects are displayed as merely points or lines. In addition, sometimes the image converted into the plane coordinate gives a sense of strangeness to the driver. Though being not so extreme as in the example described in the document, when an image of three dimensional space taken from a certain viewpoint is converted by the viewpoint conversion, objects not existing on the plane, into which the objects are converted, are not correctly converted, resulting in producing strange images.
Besides, there is another method for monitoring surroundings of a vehicle. In this method, the conversion into the plane coordinate is not performed. Alternatively, places invisible from a driver's seat as illustrated in FIG. 11 is preferentially taken into images. Then, the images are separately displayed in display frames on the display apparatus. By this method, strangeness of the images can be reduced because the viewpoint of the images is regular one. However, when the images are separately displayed as described above, a driver need to recognize positional relations between the images to grasp surroundings of the vehicle, which gives difficulties in obtaining information of surroundings of the vehicle at a glance.
A need thus exists for a monitoring system for monitoring surroundings of a vehicle which enables a driver of the vehicle to grasp the surroundings of the vehicle as wide as possible at a glance without feeling strangeness. The present invention has been made in view of the above circumstances and provides such a monitoring system for monitoring surroundings of a vehicle.